The normal mobility of the vertebral column, particularly in the cervical and lumbar regions, gives rise to major changes in the length and lumen of the spinal canal and of its volume. Its contents must therefore adapt to these changes without disturbance to their several functions. The contents are partly fluid, partly neural tissue, but also connective tissue: all with differing physical characteristics. Thus, all respond differently to changes in the space in which they are contained. When pathological changes supervene, the spinal meninges, the spinal cord and nerve roots may be adversely affected by the increases in the resting tension, by compression or bending stresses. Because the mechanical state of one region of the spine has a more than local effect, all these factors have to be considered-particularly for the interpretation of the symptoms and clinical signs of lumbosacral root pathology.